Levothyroxine is a stereoisomer of thyroxine (T4) which is degraded much more slowly and can be administered once daily in patients with hypothyroidism. Natural thyroid hormone from pigs is sometimes also used, especially for people who cannot tolerate the synthetic version. Hyperthyroidism caused by Graves' disease may be treated with the thioamide drugs propylthiouracil, carbimazole or methimazole, or rarely with Lugol's solution. Additionally, hyperthyroidism and thyroid tumors may be treated with radioactive iodine. Ethanol injections for the treatment of recurrent thyroid cysts and metastatic thyroid cancer in lymph nodes can also be an alternative to surgery.

There is little evidence whether there is a benefit from treating subclinical hypothyroidism, and whether this offsets the risks of overtreatment. Untreated subclinical hypothyroidism may be associated with a modest increase in the risk of coronary artery disease. A 2007 review found no benefit of thyroid hormone replacement except for "some parameters of lipid profiles and left ventricular function". There is no association between subclinical hypothyroidism and an increased risk of bone fractures, nor is there a link with cognitive decline.

Since 2008, consensus American and British opinion has been that in general people with TSH under 10 mIU/l do not require treatment. American guidelines recommend that treatment should be considered if the TSH is elevated but below 10 mIU/l in people with symptoms of hypothyroidism, detectable antibodies against thyroid peroxidase, a history of heart disease or are at an increased risk for heart disease.

Most children born with congenital hypothyroidism and correctly treated with thyroxine grow and develop normally in all respects. Even most of those with athyreosis and undetectable T levels at birth develop with normal intelligence, although as a population academic performance tends to be below that of siblings and mild learning problems occur in some.

Congenital hypothyroidism is the most common preventable cause of intellectual disability. Few treatments in the practice of medicine provide as large a benefit for as small an effort.

The developmental quotient (DQ, as per Gesell Developmental Schedules) of children with hypothyroidism at age 24 months that have received treatment within the first 3 weeks of birth is summarised below:

Most people with hypothyroidism symptoms and confirmed thyroxine deficiency are treated with a synthetic long-acting form of thyroxine, known as levothyroxine (-thyroxine). In young and otherwise healthy people with overt hypothyroidism, a full replacement dose (adjusted by weight) can be started immediately; in the elderly and people with heart disease a lower starting dose is recommended to prevent over supplementation and risk of complications. Lower doses may be sufficient in those with subclinical hypothyroidism, while people with central hypothyroidism may require a higher than average dose.

Blood free thyroxine and TSH levels are monitored to help determine whether the dose is adequate. This is done 4–8 weeks after the start of treatment or a change in levothyroxine dose. Once the adequate replacement dose has been established, the tests can be repeated after 6 and then 12 months, unless there is a change in symptoms. In people with central/secondary hypothyroidism, TSH is not a reliable marker of hormone replacement and decisions are based mainly on the free T level. Levothyroxine is best taken 30–60 minutes before breakfast, or four hours after food, as certain substances such as food and calcium can inhibit the absorption of levothyroxine. There is no direct way of increasing thyroid hormone secretion by the thyroid gland.

Treatment modality depends on the cause. Tumors may be removed surgically, but pituitary stalk interruption may persist. Usually, replacement of those hormones that are reduced due to failed feedback control systems will be necessary.

Radioiodine therapy with iodine-131 can be used to shrink the thyroid gland (for instance, in the case of large goiters that cause symptoms but do not harbor cancer—after evaluation and biopsy of suspicious nodules has been done), or to destroy hyperactive thyroid cells (for example, in cases of thyroid cancer). The iodine uptake can be high in countries with iodine deficiency, but low in iodine sufficient countries. To enhance iodine-131 uptake by the thyroid and allow for more successful treatment, TSH is raised prior to therapy in order to stimulate the existing thyroid cells. This is done either by withdrawal of thyroid hormone medication or injections of recombinant human TSH (Thyrogen), released in the United States in 1999. Thyrogen injections can reportedly boost uptake up to 50-60%. Radioiodine treatment can also cause hypothyroidism (which is sometimes the end goal of treatment) and, although rare, a pain syndrome (due to radiation thyroiditis).

Thyroid dyshormonogenesis (or dyshormogenetic goiter) is a rare condition due to genetic defects in the synthesis of thyroid hormones.

Patients develop hypothyroidism with a goitre.either deficiency of thyroid enzymes or inability to concentrate or ineffective binding

One particular familial form is associated with sensorineural deafness (Pendred's syndrome).

OMIM includes the following:

The goal of newborn screening programs is to detect and start treatment within the first 1–2 weeks of life. Treatment consists of a daily dose of thyroxine, available as a small tablet. The generic name is levothyroxine, and several brands are available. The tablet is crushed and given to the baby with a small amount of water or milk. The most commonly recommended dose range is 10-15 μg/kg daily, typically 12.5 to 37.5 or 44 μg.

Within a few weeks, the T and TSH levels are rechecked to confirm that they are being normalized by treatment. As the child grows up, these levels are checked regularly to maintain the right dose. The dose increases as the child grows.

Because most patients respond to steroids or immunosuppressant treatment, this condition is now also referred to as steroid-responsive encephalopathy.

Initial treatment is usually with oral prednisone (50–150 mg/day) or high-dose IV methylprednisolone (1 g/day) for 3–7 days. Thyroid hormone treatment is also included if required.

Failure of some patients to respond to this first line treatment has produced a variety of alternative treatments including azathioprine, cyclophosphamide, chloroquine, methotrexate, periodic intravenous immunoglobulin and plasma exchange. There have been no controlled trials so the optimal treatment is not known.

Seizures, if present, are controlled with typical antiepileptic agents.

Duration of treatment is usually between 2 and 25 years. Earlier reports suggested that 90% of cases stay in remission after discontinuation of treatment; however, this is at odds with more recent studies which suggest that relapse commonly occurs after initial high-dose steroid treatment. Left untreated, this condition can result in coma and death.

Hyperthyroidism is one of the most common endocrine conditions affecting older domesticated housecats. Some veterinarians estimate that it occurs in up to 2% of cats over the age of 10. The disease has become significantly more common since the first reports of feline hyperthyroidism in the 1970s. One cause of hyperthyroidism in cats is the presence of benign tumors, but the reason these cats develop such tumors continues to be studied. However, recent research published in Environmental Science & Technology, a publication of the American Chemical Society, suggests that many cases of feline hyperthyroidism are associated with exposure to environmental contaminants called polybrominated diphenyl ethers (PBDEs), which are present in flame retardants in many household products, in particular, furniture and some electronics.

The study on which the report was based was conducted jointly by researchers at the EPA's National Health and Environmental Effects Laboratory and Indiana University. In the study, which involved 23 pet cats with feline hyperthyroidism, PBDE blood levels were three times as high as those in younger, non-hyperthyroid cats. In ideal circumstances, PBDE and related endocrine disruptors that seriously damage health would not be present in the blood of any animals, including humans.

Several studies indicate canned fish, liver and giblet prepared cat food may increase risk whereas fertilizers, herbicides, or plant pesticides had no effect. Another study suggests cat litter could be a problem.

Mutations of the thyroid-stimulating hormone receptor that cause a constitutive activation of the thyroid gland cells have been discovered recently. Many other factors may play a role in the pathogenesis of the disease such as goitrogens (isoflavones such as genistein, daidzein, and quercetin) as well as the iodine and selenium content of the cat's diet.

The most common presenting symptoms are: rapid weight loss, tachycardia (rapid heart rate), vomiting, diarrhea, increased consumption of fluids (polydipsia) and food, and increased urine production (polyuria). Other symptoms include hyperactivity, possible aggression, heart murmurs, a gallop rhythm, an unkempt appearance, and large, thick claws. About 70% of afflicted cats also have enlarged thyroid glands (goiter).

The same three treatments used with humans are also options in treating feline hyperthyroidism (surgery, radioiodine treatment, and anti-thyroid drugs). The drug that is used to help reduce the hyperthyroidism is methimazole. Where drug therapy is used it must be given to cats for the remainder of their lives but this may be the least expensive option, especially for very old cats. Anti-thyroid drugs for cats are available in both pill form and in a topical gel, that is applied using a finger cot to the hairless skin inside a cat's ear. Many cat owners find this gel a good option for cats that don't like being given pills. Radioiodine treatment and surgery often cure hyperthyroidism but some veterinarians prefer radioiodine treatment over surgery because it doesn't carry the risks associated with anesthesia.

Radioiodine treatment, however, is not available in all areas for cats as this treatment requires nuclear radiological expertise and facilities as the cat's urine, sweat, saliva, and stool are radioactive for several days after the treatment requiring special inpatient handling and facilities usually for a total of 3 weeks (first week in total isolation and the next two weeks in close confinement). In the United States, the guidelines for radiation levels vary from state to state; some states such as Massachusetts allow hospitalization for as little as two days before the animal is sent home with care instructions. Surgery tends to be done only when just one of the thyroid glands is affected (unilateral disease); however, following surgery, the remaining gland may become overactive. As in people, one of the most common complications of the surgery is hypothyroidism.

In a study of 1,034 symptomatic adults, Sheehan syndrome was found to be the sixth most frequent etiology of growth hormone deficiency, being responsible for 3.1% of cases (versus 53.9% due to a pituitary tumor).

People with autoimmune hyperthyroidism should not eat foods high in iodine, such as edible seaweed and kelps.

From a public health perspective, the general introduction of iodized salt in the United States in 1924 resulted in lower disease, goiters, as well as improving the lives of children whose mothers would not have eaten enough iodine during pregnancy which would have lowered the IQs of their children.

Thyroid dysgenesis or thyroid agenesis is a cause of congenital hypothyroidism where the thyroid is missing, ectopic, or severely underdeveloped.

It should not be confused with iodine deficiency, or with other forms of congenital hypothyroidism, such as thyroid dyshormonogenesis, where the thyroid is present but not functioning correctly.

Congenital hypothyroidism caused by thyroid dysgenesis can be associated with PAX8.

Pregnant women who are positive for Hashimoto's thyroiditis may have decreased thyroid function or the gland may fail entirely. If a woman is TPOAb-positive, clinicians can inform her of the risks for themselves and their infants if they go untreated. "Thyroid peroxidase antibodies (TPOAb) are detected in 10% of pregnant women," which presents risks to those pregnancies. Women who have low thyroid function that has not been stabilized are at greater risk of having an infant with: low birth weight, neonatal respiratory distress, hydrocephalus, hypospadias, miscarriage, and preterm delivery. The embryo transplantion rate and successful pregnancy outcomes are improved when Hashimoto's is treated. Recommendations are to only treat pregnant women who are TPOAb-positive throughout the entirety of their pregnancies and to screen all pregnant women for thyroid levels. Close cooperation between the endocrinologist and obstetrician benefits the woman and the infant. The Endocrine Society recommends screening in pregnant women who are considered high-risk for thyroid autoimmune disease.

Thyroid peroxides antibodies testing is recommended for women who have ever been pregnant regardless of pregnancy outcome. "...[P]revious pregnancy plays a major role in development of autoimmune overt hypothyroidism in premenopausal women, and the number of previous pregnancies should be taken into account when evaluating the risk of hypothyroidism in a young women ["sic"]."

Treatment of a thyroid nodule depends on many things including size of the nodule, age of the patient, the type of thyroid cancer, and whether or not it has spread to other tissues in the body.

If the nodule is benign, patients may receive thyroxine therapy to suppress thyroid-stimulating hormone and should be reevaluated in 6 months. However, if the benign nodule is inhibiting the patient's normal functions of life; such as breathing, speaking, or swallowing, the thyroid may need to be removed.

Sometimes only part of the thyroid is removed in an attempt to avoid causing hypothyroidism. There's still a risk of hypothyroidism though, as the remaining thyroid tissue may not be able to produce enough hormones in the long-run.

If the nodule is malignant or has indeterminate cytologic features, it may require surgery. A thyroidectomy is a medium risk surgery that can result complications if not performed correctly. Problems with the voice, nerve or muscular damage, or bleeding from a lacerated blood vessel are rare but serious complications that may occur. After removing the thyroid, the patient must be supplied with a replacement hormone for the rest of their life. This is commonly a daily oral medication prescribed by their endocrinologist.

Radioactive iodine-131 is used in patients with papillary or follicular thyroid cancer for ablation of residual thyroid tissue after surgery and for the treatment of thyroid cancer. Patients with medullary, anaplastic, and most Hurthle cell cancers do not benefit from this therapy. External irradiation may be used when the cancer is unresectable, when it recurs after resection, or to relieve pain from bone metastasis.

Life long hormone replacement therapy for the hormones that are missing.

GH deficiency is treated by replacing GH with daily injections under the skin or into muscle. Until 1985, growth hormone for treatment was obtained by extraction from human pituitary glands collected at autopsy. Since 1985, recombinant human growth hormone (rHGH) is a recombinant form of human GH produced by genetically engineered bacteria, manufactured by recombinant DNA technology. In both children and adults, costs of treatment in terms of money, effort, and the impact on day-to-day life, are substantial.

GH treatment is not recommended for children who are not growing despite having normal levels of growth hormone, and in the UK it is not licensed for this use. Children requiring treatment usually receive daily injections of growth hormone. Most pediatric endocrinologists monitor growth and adjust dose every 3–6 months and many of these visits involve blood tests and x-rays. Treatment is usually extended as long as the child is growing, and lifelong continuation may be recommended for those most severely deficient. Nearly painless insulin syringes, pen injectors, or a needle-free delivery system reduce the discomfort. Injection sites include the biceps, thigh, buttocks, and stomach. Injection sites should be rotated daily to avoid lipoatrophy. Treatment is expensive, costing as much as US $10,000 to $40,000 a year in the USA.

Ideally a woman who is known to have hyperthyroidism should seek pre-pregnancy advice, although as yet there is no evidence for its benefit. Appropriate education should allay fears that are commonly present in these women. She should be referred for specialist care for frequent checking of her thyroid status, thyroid antibody evaluation and close monitoring of her medication needs. Medical therapy with anti-thyroid medications is the treatment of choice for hyperthyroidism in pregnancy.Methimazole and propylthiouracil (PTU) are effective in preventing pregnancy complications by hyperthyroidism. Surgery is considered for patients who suffer severe adverse reactions to anti-thyroid drugs and this is best performed in the second trimester of pregnancy. Radioactive iodine is absolutely contraindicated in pregnancy and the puerperium. If a woman is already receiving carbimazole, a change to propylthiouracil (PTU) is recommended but this should be changed back to carbimazole after the first trimester. This is because carbimazole can rarely be associated with skin and also mid line defects in the fetus but PTU long term also can cause liver side effects in the adult. Carbimazole and PTU are both secreted in breast milk but evidence suggests that antithyroid drugs are safe during lactation. There are no adverse effects on IQ or psychomotor development in children whose mothers have received antithyroid drugs in pregnancy.

Current guidelines suggest that a pregnant patient should be on PTU during the first trimester of pregnancy due to lower tetragenic effect and then be switched to methimazole during the second and third trimester due to lower liver dysfunction side effects.

Hypothyroidism caused by Hashimoto's thyroiditis is treated with thyroid hormone replacement agents such as levothyroxine, triiodothyronine or desiccated thyroid extract. A tablet taken once a day generally keeps the thyroid hormone levels normal. In most cases, the treatment needs to be taken for the rest of the person's life. In the event that hypothyroidism is caused by Hashimoto's thyroiditis, it may be recommended that the TSH levels be kept under 3.0 mIU/L.

Pickardt syndrome (also Pickardt's syndrome or Pickardt–Fahlbusch syndrome) denotes a rare form of tertiary hypothyroidism that is caused by interruption of the portal veins connecting hypothalamus and pituitary.

It was characterized in 1972 and 1973.

Myxedema psychosis, more colloquially known as myxedema madness, is a relatively uncommon consequence of hypothyroidism, such as in Hashimoto's thyroiditis or in patients who have had the thyroid surgically removed and are not taking thyroxine. A chronically under-active thyroid can lead to slowly progressive dementia, delirium, and in extreme cases to hallucinations, coma, or psychosis, particularly in the elderly. It was first recognized by Dr Richard Asher of London in 1949.

Treatment is via the standard treatment for hypothyroidism with thyroxine replacement. Oral T, or in especially acute cases liothyronine, a sodium salt of T. Hormone replacement in these patients usually reverses the psychotic symptoms, but may not help with cognitive deficits caused by changes in metabolic activity in the CNS.

The "myxedema" part of the name simply refers to the non-pitting edema common to hypothyroidism.

Treatment is as with hypothyroidism, daily thyroxine(T4) and/or triiodothyronine(T3).